The present invention generally relates to the field of repairing composite structures. In particular, the invention relates to doubler repair assemblies for composite arcuate flanges.
Composite structures that are formed of a thermoplastic material such as polyamideimide or polyetherimide are resistant to high temperatures and engine fluids. Thus, these composite structures can be exposed to extreme environments, such as hot engine oils in aircraft engines, which typically cause degradation of mechanical properties of most other elastomeric materials. However, while the composite structure may be temperature resistant, the composite structure and its parts are subjected to thermal stresses when rigidly attached to metal structures with significantly different coefficients of thermal expansion, causing cracks in the weaker composite structure. For example, arcuate flanges formed of composite materials can easily form cracks radiating from apertures in the flanges where the bolts are positioned. If left unattended, the thermal stresses of the environment may cause the crack to continue to extend to the edge of the flange and cause the entire flange to break.
Traditional repair techniques are limited to flange reconstruction techniques using fiberglass/epoxy laminates or either solvent or thermal welded replacement sections. Most of these current repair techniques result in a repair assembly that has inferior mechanical properties when compared to the original structure and also do not account for thermal disparity between parts. One method of repairing the damaged composite structure is to replace the entire structure. However, this can be a costly process, particularly if only a single aperture or only a small percentage of the composite structure is damaged. Another option is to restore only a piece or segment of the flange to a workable condition using a splint or doubler assembly that is applied to the damaged portion of the structure. Additional methods of repairing a damaged composite structure are described in U.S. Pat. No. 5,876,651 and U.S. Pat. No. 5,965,240, issued to Blackburn et al.